1. Field of the Invention
This invention relates to an optical communications system, and in particular to a line monitoring arrangement for early detection of faults in such a system.
2. Related Art
If an installed optical fibre is to be tested for an apparent or suspected fault, the way in which this is presently performed is by utilising an Optical Time Domain Reflectometer (OTDR). An OTDR comprises a pulse source, usually a high power laser, from which a single pulse is launched into the fibre to be tested, and light returning to the launch end of the fibre is monitored. This returned light always includes backscattered light (that is to say light that is reflected back within the fibre's numerical aperture due to Rayleigh scattering) and may include light reflected at fibre discontinuities such as fibre breaks, fibre splices or points in the light path where refractive index differences occur. Falls in the returned light level are indicative of increased attenuation in fibre splices and connectors, and peaks in the returned light level are indicative of reflections from breaks in the fibre. The distance of a given feature from the launch end of the fibre is known from the time delay of the associated return signal. Once a period of time sufficient to receive all detectable returned light has passed, a further pulse may be launched into the fibre. The pulse width may be varied for different dynamic range or resolution requirements.
A particular limitation of using an OTDR of this type is that the optical communications system on the fibre has to be disconnected, or at least discontinued, both to permit the connection of the OTDR, and also to prevent the system and the OTDR signals from interfering with one another. The result of this limitation is that an OTDR tends to be utilised only after a failure occurs, and cannot be used for constant line surveillance simultaneously with data transmission.
The specification of International patent application No. GB90/01250 describes a line monitoring arrangement of an optical fibre communications system carrying system data. The monitoring arrangement comprises a test sequence generator for generating a sequence of test pulses, means for repeatedly launching the sequence of test pulses into a first end of a transmission line forming part of the communications system, and a correlator for correlating signals received at the first end of the transmission line with the delayed sequence of test pulses to identify returned test pulse sequences.
This line monitoring arrangement can be operated, while data is being transmitted, by time multiplexing test pulses along with the system data pulses. Unfortunately, restricting the launch of test pulses in this way limits the processing gain of the monitoring arrangement, and hence its effectiveness.